[flang] add preliminary README file

Original-commit: flang-compiler/f18@1fc60916fbc3339056fe889e8ac50a2906387c49
Reviewed-on: https://github.com/flang-compiler/f18/pull/6
Tree-same-pre-rewrite: false
1 file changed
tree: a21e536d7eeda915ebc334b8d7555d75cb1694c3
  1. flang/